someone who works for free is also a volunteer.
An office worker might not be a volunteer,
If work without pay is volunteer, and with pay is slavery, then we are in a place I've never been. Or heard of for that matter.
If you replace "pay" with "coersion" it makes more sense. To own slaves, one needs to compensate them with food and housing, or some kind of "pay" that keeps them alive and one's slave. Pay has evolved in the time since serfs. To call modern wage slaves simply 'slaves' may be a bit of an overstatement, but it still rings true enough to get my point across.
Anyone familiar with sharecroppers?
Replacing pay with coercion is similar to replacing execution with exoneration.
It changes the meaning to the polar opposite.
One is force, the other is enticement. One is compulsion the other opportunity and choice.
Consider instead that being an employee is the step after generalized schooling.
It is that part of your training to be an entrepreneur, where you get paid to learn.
Its like graduate school for those that want to do things rather than research.
Those that graduate and become entrepreneurs can then engage in training their own employees.
In practice, what you describe is the exception to the rule within a state capitalist framework. Dependency on employers prevents many from ever coming far enough out of debt to do what they want.
I have a hard time with equating pay to exoneration or choice because reliance on any paycheck does not let you all the way out of the state or capitalist's control. Pay is giving a man a fish and assuming he has means to stockpile fish until he can learn to fish.